Jacobs,  J., Bauer, W., Fanning, C.M., 2003. New age constraints for Grenville-age metamorphism in western central Dronning Maud Land (East Antarctica), and implications for the palaeogeography of Kalahari in Rodinia 

New SHRIMP zircon data from Gjelsvikfjella and Mühlig-Hoffmann-Gebirge (East Antarctica) indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the northeastern continuation of the Namaqua-Natal-Maud Belt. Crystallisation ages of meta-igneous rocks range between c. 1150 to 1100 Ma, with little inheritance recorded. Metamorphic zircon overgrowth during high-grade metamorphism is dated between c. 1090 to 1050 Ma. Both, the crystallisation ages and the metamorphic overprint are similar to U-Pb data from a number of areas along a c. 2000 km stretch from Natal in South Africa to central Dronning Maud Land. The basement underwent in part strong high-grade reworking during the collision of East- and West-Gondwana at c. 530 Ma. The timing of Grenville-age metamorphism has important implications for the position of Kalahari in Rodinia.  It also questions that Coats Land is part of the Maud Belt, since the undeformed volcanic rocks of Coats Land are older than the main metamorphism within the Maud Belt, and therefore must rest on older basement. This interpretation explains why the pole of Coats Land at c. 1110 Ma differs from the Kalahari poles by 30°, i.e. Coats Land had not yet amalgamated to Kalahari. On the other hand, the palaeopoles from Coats Land and Laurentia at 1110 Ma are identical within error. Thus, Coats Land could have been part of Laurentia prior to the final amalgamation of Rodinia, the Namaqua-Natal-Maud Belt could have been a part of the Grenville Belt and the entire Kalahari Craton could indeed have opposed Laurentia on its eastern side.

Jacobs, J., Breitkreuz, C., 2003. Zircon and apatite fission-track thermochronology of Late Carboniferous volcanic rocks of the NE German Basin

Zircon and apatite fission-track analyses from Late Carboniferous felsic volcanic rocks of the NE German Basin (Halle area and Friedland drilling) reveal at least two major post-emplacement thermal events. After initial cooling at ca. 300 Ma, the volcanic pile underwent a major thermal event at ca. 200 Ma that reached in most places ca. 250-280 °C and led to the new growth of clay minerals. This event is recorded in the zircon fission-track data and can be related to Jurassic-Triassic rifting in Europe. Another thermal event is recorded in the apatite samples at ca. 100 Ma. A close correlation is observed between apatite fission-track age, texture, and sample alteration. Coarse-grained samples are stronger altered and have younger fission-track ages. Hydrothermal alteration coupled with advective heating probably caused these Upper Cretaceous fission-track ages. The latter event can be related to block faulting and inversion of the European plate during the early stages of the Alpine orogeny.

Jacobs,  J., Bauer, W., Fanning, C.M., 2003. Late Neoproterozoic/Early Palaeozoic events in central Dronning Maud Land and significance for the southern extension of the East African Orogen into East Antarctica

New SHRIMP zircon data from Gjelsvikfjella and Mühlig-Hoffmann-Gebirge indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the northeastern continuation of the Namaqua-Natal-Maud Belt. High-grade overprinting occurred between c. 560 to 490 Ma. This period is considered to bracket the collision of E- and W-Gondwana along the East African/Antarctic Orogen (EAAO) and also the subsequent collapse of the orogen. The onset of collision probably predates our oldest dates. We can clearly differentiate between an early compressional stage (Pan-African I) and a later dilational episode (Pan-African II). The Pan-African I event is characterised by the intrusion of leucogranites, and the formation of medium-U metamorphic zircon overgrowth. A sample of a syn-tectonic leucogranite gave a well constrained crystallisation age of 558.4 ± 5.6 Ma. These rocks are intensely deformed and have formed a second Gt-bearing leucosome after their emplacement. Identical ages come from low-U metamorphic zircons and low-U rims from another leucosome at Festninga (557 ± 13 Ma). The Pan-African II event (c. 530 and 490 Ma) is characterised by widespread melting and only little deformation. It is associated with charnockite magmatism and charnockitisation and high temperature metamorphism. Wide, very high-U zircon rims form during this period. The early Pan-African II overprint between c. 530-520 Ma is recorded in almost all samples. An undeformed mafic dyke with a zircon crystallisation age of 523.2 ± 4.8 indicates that there is no or minor deformation after its intrusion. This strongly metasomatised dyke however also indicates that although deformation ceased, intense fluid circulation continued. The youngest crystallisation ages come from a post-tectonic granite sheet which yields a crystallisation age of 486.9 ± 3.8 Ma. Final cooling to temperatures below c. 600 °C is recorded by a titanite age of c. 483 ± 11 Ma from the Stabben gabbro. Late to post-tectonic granitoids are voluminous and intrude over a period of c. 30 Ma,  between c. 520 – 490 Ma. They are alkaline in composition and can be classified as A2- type. This indicates that they probably formed by melting of a tonalitic to granodioritic lower crust in the cause of crustal thickening, or from underplated lower crust.

Jacobs, J., Fanning, C.M., Bauer, W., 2003. Timing of Grenville-age vs. Pan-African medium- to high grade metamorphism in western Dronning Maud Land (East Antarctica) and significance for correlations in Rodinia and Gondwana

Heimefrontfjella in western Dronning Maud Land, East Antarctica, is part of the c. 1.1 Ga Namaqua–Natal–Maud Belt. It is situated at an important location, both for Rodinia reconstructions and at the western orogenic front of the Late Neoproterozoic/Early Palaeozoic East African/Antarctic Orogen. Six basement samples help to precisely date the Grenville-age metamorphism between c. 1090 and 1060 Ma. These dates indicate that palaeomagnetic data from the Coats Land Block to the south of the Maud Belt cannot be taken to support the contiguity of the Coats Land/Maud/Grunehogna continental fragment and Kalahari during Late Mesoproterozoic times. A comparison of aeromagnetic data and crustal provinces indicates that the Coats Land basement is likely to be composed of pre-Mesoproterozoic basement that was differentially reworked at c. 500 Ma within the East African/Antarctic Orogen. The western orogenic front of the East African/Antarctic Orogen is exposed as the Heimefront Shear Zone. Zircons from this shear zone show a thin, low-U, reaction zone that probably resulted from mylonitisation at amphibolite facies conditions. The rims were too thin to be analysed. However, only a few kilometres to the E of the shear zone, c. 500 Ma Pan-African zircon overgrowth is present, clearly indicating intense Late Neoproterozoic/Early Palaeozoic reworking of the Grenville-age basement and allowing the direct dating of this younger overprint. High grade reworking must have been followed by rapid cooling, as shown by a number of c. 500 Ma K–Ar and Ar–Ar hornblende and mica ages. Whereas the western orogenic front of the East African/Antarctic Orogen is exposed as the Heimefront Shear Zone, the eastern orogenic front might be represented by the Otter Highland Thrust in the Shackleton Range, with variably reworked crust up to granulite facies in between these two structural discontinuities.

Precamb. Res., 125, 1-20

Bauer, W., Fielitz, W., Jacobs, J., Fanning, C.M. & Spaeth, G., 2003. Mafic Dykes from Heimefrontfjella and implications for the post-Grenvillian to pre-Pan-African geological evolution of western Dronning Maud Land (Antarctica)

Two groups of geochemical different dykes have been identified in the Grenville-aged basement of Heimefrontfjella. The first group comprises dykes of continental tholeiite composition which probably intruded during the final stage of indentation of the Kaapvaal-Grunehogna Craton into Laurentia. One dyke of this group yielded an U-Pb zircon SHRIMP age of 1033 ±7 Ma. The second group has an E-type MORB composition and may be related to ocean floor basalts of the Mozambique Ocean between East and West Gondwana. A preliminary U-Pb SHRIMP age of 586 ±7 Ma for a single zircon crystal was obtained from a dyke of the second group. During the Pan-African orogeny both dyke groups underwent metamorphism and tectonism at different grades: up to amphibolite-facies in the eastern and southern Heimefrontfjella, and at greenschist-facies in the western and northern Heimefrontfjella. The older dykes may be correlated with the Equeefa suite of southern Natal whereas the younger dyke group is not correlatable with any known mafic intrusions or lava flows in adjacent regions.

Bauer, W., Jacobs, J., Fanning, C.M. & Schmidt, R., 2003. Late Mesoproterozoic arc and back-arc volcanism in the Heimefrontfjella (East
Antarctica) and implications for the palaeogeography at the southeastern margin of the Kaapvaal-Grunehogna Craton

The nunataks of Heimefrontfjella in Dronning Maud Land (Antarctica) are dominantly composed of juvenile Late Mesoproterozoic rocks that were metamorphosed and deformed at about 1080 and 500 Ma. Three discontinuity-bound terranes have been identified, namely the Kottas, Sivorg and Vardeklettane terranes. In the Sivorg terrane the basement is mainly made up of fine-grained banded felsic and mafic gneisses, representing a metamorphosed bimodal volcanic sequence. In contrast, the basement in the Kottas terrane is composed of banded gneisses of tonalitic to dioritic composition and calc-alkaline metaplutonic rocks. The metamorphic rocks of the Vardeklettane terrane are granulites of supra- to infracrustal origin.
Geochemical data characterize the protoliths of the bimodal sequence in the Sivorg terrane as tholeiitic basalts with MORB signature, alternating with high K-rhyolites derived from a highly fractionated granitic magma. Four U-Pb zircon SHRIMP ages prove a magmatic activity in the Sivorg terrane at least from ~1160 to 1090 Ma. The orthogneisses from the Kottas terrane plot in the field of volcanic arc granites. Their protoliths may be derived from a calc-alkaline magmatic suite. According to recent reconstructions of the Kalahari continent the basement of the Kottas terrane is an Antarctic counterpart of the Mzumbe terrane in Natal. Our data allow a reconstruction of a subduction-related volcanic arc (Kottas Arc) along a southerly directed subduction zone which existed from ~1200 to 1100 Ma, finally colliding with the southeastern margin of the Kaapvaal-Grunehogna craton. South of the volcanic arc, a back-arc basin (Sivorg terrane) developed during latest Mesoproterozoic times. Undeformed calc-alkaline granites in the northernmost Heimefrontfjella point to a final northward directed subduction of the oceanic lithosphere of the back-arc basin which led to the collision of the Coats Land block with the Kaapvaal-Grunehogna craton.

Bauer, W., Thomas, R.J. & Jacobs, J., 2003. Proterozoic-Cambrian History of Dronning Maud Land in the context of Gondwana assembly

Dronning Maud Land contains a fragment of an Archaean craton covered by sedimentary and magmatic rocks of Mesoproterozoic age, surrounded by a Late Mesoproterozoic metamorphic belt. Tectonothermal events at the end of the Mesoproterozoic and at Late Neoproterozoic/Cambrian times ("Pan-African") have been proved within the metamorphic belt. In western Dronning Maud Land a juvenile Mesoproterozoic basement was accreted to the craton at ~ 1.1 Ga. Mesoproterozoic rocks were also detected by zircon SHRIMP dating of gneisses in central Dronning Maud Land, followed by a long hiatus for which geochronological data are lacking, an amphibolite to granulite facies metamorphism and syntectonic granitoid emplacement of Pan-African age have been dated. During this orogeny older structures were completely overprinted in a sinistral tranpressive deformation regime, leading to the mainly coast-parallel tectonic structures of the East Antarctic Orogen. Putting Antarctica back in its Gondwana position, the East Antarctic Orogen continues northward in E-Africa as the East African Orogen, whereas a connection to the marginal Ross Orogen at the Pacific margin of East Antarctica is suggested along the Shackleton Range. The East Antarctic/East African Orogen resulted from closure of the "Mozambique Ocean" and collision of West and East Gondwana, i.e. western Dronning Maud Land was part of West Gondwana. During this collision the lithospheric mantle probably delaminated, allowing the asthenosphere to underplate the continental crust and producing heat for the voluminous, typically anhydrous Pan-African granitoids of central Dronning Maud Land.

Jacobs, J., Klemd, R., Fanning, C.M., Bauer, W. & Colombo, F., 2003. Extensional collapse of the Late Neoproterozoic/Early Paleozoic East Antarctic orogen: evidence from central Dronning Maud Land

The East Antarctica Orogen resulted from the continent-continent collision of E- and W-Gondwana, or parts thereof, during the Pan-African event at c. 600-510 Ma. The collision overprinted large areas of older, mainly Mesoproterozoic crust up to granulite facies grade. The collision history is well documented by folding and thrusting, isothermal decompression and metamorphic zircon growth at c. 580-560 Ma (Pan-African I). The convergence was succeeded by an extensional phase, probably representing orogenic collapse of the East Antarctic Orogen. This Pan-African II event at c. 530-510 Ma is characterised by large-scale extensional structures, finally resulting in the posttectonic intrusion of voluminous A2-type granitoids. In central Dronning Maud Land the Pan-African II event starts with the intrusion of syntectonic igneous rocks within an overall extensional setting. Two new SHRIMP data from gabbro zircons of the Zwiesel gabbro give ages of 521 ± 5.6 Ma and 527 ± 5.1 Ma. These ages are interpreted as crystallisation ages and confirm the interpretation that the gabbro was emplaced early during the Pan-African II event. The gabbro was intruded by a network of leucogranite dykes and veins. Whereas the gabbro appears entirely undeformed, the leucogranite dykes are strongly mylonitised, along extensional shear zones indicating pronounced strain partitioning of the gabbro complex. Within the leucogranite mylonites large tension gashes have developed during mylonitization, indicating very high strain rates. Quartz c-achses orientations from quartz of the tension gashes show a distinct cross-girdle, that formed during pure shear deformation. Fluid inclusion data from the leucogranite mylonites and the associated tension gashes mainly reveal re-crystallization related intracrystalline CO2-dominant inclusions with relatively low densities of < 1 g/cm3.  The fluid inclusion data are interpreted to represent the last stages of a retrograde PT-path that is characterised by simultaneous cooling and decompression during extensional exhumation, probably succeeding the collapse of overthickened crust. A comparable orogenic collapse of the East African/Antarctic Orogen is reported from other parts of the orogen, such as from western Madagascar and the northern Arabian-Nubian Shield.

In: Yoshida, M., Windley, B.F. & Dasgupta, S. (eds.)
Proterozoic East Gondwana: Supercontinent Assembly and Breakup
Geol. Soc. Lond., Spec. Publ., 206, 271-287

Jacobs, J. & Bauer, W., 2001. Gjelsvikfjella, Mühlig-Hoffmann-Gebirge: Another piece of the East Antarctic Orogen?

Structural field work was carried out in eastern Gjelsvikfjella and western Mühlig-Hoffmann-Gebirge during the austral summer 1999/00 in order to check whether theses areas are part of the East Antarctic Orogen. First results point to a complex tectonic, metamorphic and magmatic history of the study areas. The probably oldest rocks in the area are a sequence of grey migmatic gneiss of tonalitic to trondhjemitic composition, containing abundant boudinaged remnants of mafic dykes. This sequence was intensely deformed together with various supracrustal rocks such as sil-gneiss, calcsilicates and marbles. The whole complex underwent different phases of high-.grade metamorphism, migmatisation and polyphase tectonism. A distinct suite of non-migmatic meta-granitoids (now augen-gneisses) form important time markers within the complex structural evolution. Late-to post-tectonically, parts of the study area were intruded by Pan-African granitoids including charnockites, granites and gabbros.

Jacobs, J. & Thomas, R.J., 2002. A titanite fission-track profile across the SE Archean Kaapvaal craton and the Mesoproterozoic Natal Metamorphic Province, South Africa: evidence for differential cryptic Meso- to Neoproterozoic tectonism
 
Titanite fission track (TFT) data are presented from samples taken at regular intervals along a ~250 km long traverse the c. 1.1 Ga Natal Metamorphic Province into the Archaean Kaapvaal Craton, SE South Africa. The TFT traverse was intended to distinguish any differences in the low-temperature (280°C) cooling trajectories between the two provinces and to identify so far unrecorded exhumation events. TFT age data in 4 groupings were obtained namely at 1100 ± 200 Ma, 670 ± 100 Ma, 520± 50 Ma and 170 ± 15 Ma. In addition to these main groupings, the older group contains a few single grain ages of Paleoproterozoic age in the northernmost craton sample, from near Piet Retief. Samples from the craton, and the northernmost sample from the Natal Metamorphic Province, show a concentration of Grenville-age (c.1100 Ma) single grain ages. They reveal a cooling event that indicates that the craton margin was affected during the Mesoproterozoic to a larger extent then previously thought. Possibly, the dates reflect the timing of differential uplift of the rigid craton in response to loading by the overthrusting of the Tugela nappes some 200 km to the south during the Mesoproterozoic collision event of the Natal Metamorphic Province. Alternatively, the data may suggest that the Tugela nappes extended far further north than previously thought, and the dates represent uplift from their unroofing. The age group of 670 ± 100 Ma is interpreted to represent mixed ages, with some titanite grains having predominantly Grenville-age age components and others having mainly Pan-African age components. Generally, the intensity of Pan-African overprint increases southwards. The southern craton samples and all samples from the Natal Metamorphic Province  show a concentration of Pan-African single grain ages of c. 520 ± 50 Ma. These dates are considered to represent fairly rapid uplift of the western foreland during the Pan-African collision of East and West Gondwana which took place far to the east. Finally, samples taken from the NE part of the traverse give Jurassic dates, taken to represent thermal re-setting during the Karoo volcanic event as a result of the break-up of Gondwana. The TFT data presented, thus aim to demonstrate the power of medium to low closure temperature geochronometer to reveal and distinguish younger, largely unseen but nevertheless important tectonic events.

J. Afr. Earth Sci., 33, 323-333

Jacobs, J. & Thomas, R.J., 2002. The Mozambique Belt from an East Antarctic perspective

A review of new structural and geochronological data from East Antarctica indicates that the southern, high-grade part of the c. 500 Ma Pan-African Mozambique Belt extended southwards into Dronning Maud Land (DML) within Gondwana. The protolith ages in both the Mozambique Belt in southern East Africa and in DML are c. 1.2 to 1.0 Ga, recording an earlier (Grenvillian-Namaquan) phase of crustal growth and orogeny. The geochronological evidence shows that intense Pan-African deformation in DML was accompanied by widespread granitoid and anorthosite magmatism at c. 600 Ma, followed by polyphase high grade metamorphism between c. 580 and 515 Ma. This situation is entirely analogous to central parts of the Mozambique Belt in southern East Africa. The marginal zones of the N-S trending Pan-African orogenic belt, which constitutes the collision zone between East and West Gondwana, are marked by extensive oblique sinistral strike slip zones along the length of the Mozambique Belt in East Africa. Certain domains within the Mozambique Belt, such as the Lurio Block and parts of central DML, trend SW-NE (African azimuth), oblique to the general N-S strike of the southern Mozambique Belt. The Lurio ? central Dronning Maud Land Block could represent a Grenville-age microplate that lay within the southern Mozambique Ocean and collided with W-Gondwana at c. 650 Ma. This would explain the assembly of E- and W-Gondwana at c. 570 Ma and the early granulite facies rocks of c. 650 to 600 Ma age in Tanzania.

Golynsky, A. & Jacobs, J., 2001. Grenville-age versus Pan-African magnetic anomaly imprints in western Dronning Maud Land, East Antarctica

In this paper we examine aeromagnetic data from a part of the western margin of the Pan-African East Antarctic Orogen. The East Antarctic Orogen represents the southern continuation of the East African orogen that together formed during the collision of E- and W-Gondwana during Late Neoproterozoic/Early Paleozoic times (c. 580-515 Ma). The western margin of the East Antarctic Orogen is exposed in Heimefrontfjella, western Dronning Maud Land, where the western front of this orogen crops out as the Heimefront Shear Zone. Crust west of the Heimefront Shear Zone has typical Mesoproterozoic to Early Neoproterozoic (Grenville-age) K-Ar and Ar-Ar mineral cooling ages and magnetic anomalies are broad, of high-amplitude, elongate, craton-parallel with long wavelengths. East of the Heimefront Shear Zone, K-Ar and Ar-Ar mineral cooling ages range between c. 570-470 Ma and the magnetic anomaly pattern is entirely different. Here, a large magnetic low persists, that is overprinted by small-scale anomalies which are oriented parallel to the regional Pan-African structural trends at a high angle to the Mesoproterozoic anomalies. Thus, the Pan-African tectono-thermal overprint has caused a fundamental redistribution of magnetic minerals. The data show that the combination of aeromagnetic mapping along with detailed field work is a powerful method to delineate the extent of the East Antarctic orogen in poorly exposed Antarctica. Aeromag. Image

Jacobs, J., Hansen, B.T., Henjes-Kunst, F., Thomas, R.J., Weber, K., Armstrong, R.A. & Cornell, D.H., 1999. New age constraints on the Proterozoic/Lower Paleozoic evolution of Heimefrontfjella, East Antarctica, and its bearing on Rodinia/Gondwana correlations

New conventional and SHRIMP U-Pb zircon ages, K-Ar and Ar-Ar mineral ages and Rb-Sr data for rocks of the Kottas and Sivorg Terrane of Heimefrontfjella, western Dronning Maud Land (East Antarctica) are presented. The structure of Heimefrontfjella is characterized by a major shear zone, the Heimefront Shear Zone, that separates pervasively Pan-African overprinted crust to the SE from little-affected crust to the NW. Our new geochronological data confirm earlier findings that Heimefrontfjella lacks any Pan-African magmatism, and justify a previous interpretation that it represents a marginal zone of a wider Pan-African belt that dissects East Antarctica further to the east. Five new U-Pb zircon ages from meta-igneous rocks range from c. 1150-1050 Ma. Tonalitic orthogneisses from the juvenile Kottas arc were dated at 1130 ± 17 Ma. This age is very similar to dates reported from central Dronning Maud Land, the adjacent Falkland Microplate and the Natal belt (South Africa). The Sivorg Terrane has given no record of any pre- Pan-African K-Ar, Ar-Ar and Rb-Sr ages; all ages are younger then c. 570 Ma, indicative of pervasive Pan-African reworking. In contrast, the Kottas Terrane, which is separated from the Sivorg Terrane by the Heimefront Shear Zone, generally gives Grenville-age mineral cooling ages of c. 1000 Ma. During the Pan-African event, the Kottas Terrane was only affected by discrete shear zones, which are generally less then 1 m wide. Our new data thus confirm that the Heimefront Shear is a major discontinuity that juxtaposed disparate basement blocks (terranes) that were differentially overprinted during the Pan-African event.

Jacobs, J., Thomas, R. J., Armstrong, R. A. & Henjes-Kunst, F., 1999. Age and thermal evolution of the Mesoproterozoic Cape Meredith Complex, West Falkland

New U-Pb SHRIMP zircon and ⁴⁰Ar-³⁹Ar mineral ages are reported for rocks of the Cape Meredith Complex (CMC), West Falkland. Felsic (rhyolitic) gneisses of the oldest Big Cape Formation (BCF) give a SHRIMP zircon date of 1118 ± 8 Ma, interpreted as the date of extrusion. Three phases of granitoid intrude the BCF namely, oldest, granodiorite orthogneiss (G1), syntectonic feldspar porphyroblastic granite gneiss (G2) and post-tectonic pink leucocratic granite (G3). These granitoids gave SHRIMP dates of ~1090, 1067 ± 9 and 1003 ± 14 Ma respectively. Metamorphic overgrowths on the felsic gneisses were SHRIMP dated at ~1000 Ma, coeval with the G3 granite, whilst a date of 1135 ± 11 Ma from inherited cores in G2 zircons were probably derived from a slightly older component of the BCF. ⁴⁰Ar-³⁹Ar plateau ages from hornblende separates from amphibolite gneisses of the BCF (1009 ± 14 and 1015 ± 6 Ma), along with muscovite (989 ± 3 Ma) and biotite (989 ± 7 Ma) separated from G3 pegmatites show that the complex cooled relatively rapidly to below 350°C, with no evidence of any Pan-African (~500 Ma) thermal overprinting. These data show that the older rocks of the CMC are significantly younger than the equivalent rocks of the Natal Metamorphic Province, SE Africa (~1200 Ma), though they are comparable in age to those of Western Dronning Maud Land (East Antarctica), areas which Gondwana reconstructions place west and east of the Falkland Microplate respectively. This suggests that the older rocks of the CMC may represent younger outboard arc terranes relative to those exposed in Natal. However, the syn- to post-tectonic granites of the CMC are comparable in age in all three areas (1070 to 1000 Ma), suggesting similar collision and post-collision histories throughout the entire region. The lower temperature history of the CMC down to 350°C at ~1000 Ma, as revealed by the ⁴⁰Ar-³⁹Ar data, are comparable to data from Natal, but do not show the Pan-African (~500 Ma) overprint which is characteristic of much of western Dronning Maud Land. This situation is consistent with the proposed position of the Falkland Microplate between SE Africa and East Antarctica in Gondwana, where a general eastward increase in the intensity of the Pan-African thermal effects has been recorded.

Jacobs, J., 1999. Neoproterozoic/Lower Paleozoic events in central Dronning Maud Land (East Antarctica)

New geochronological data indicate that central Dronning Maud Land in East Antarctica underwent polyphase Neoproterozoic/Lower Palaeozoic metamorphism that can be correlated with the final amalgamation of E- and W-Gondwana. Central Dronning Maud Land most probably represents part of the southern continuation of the Mozambique Belt into E-Antarctica. The Neoproterozoic/Lower Palaeozoic metamorphism is preceded by a period of anorogenic anorthosite-charnockite magmatism at c. 600 Ma. Polyphase metamorphism is recorded from c. 580 to 515 Ma. Voluminous syntectonic magmatism has been documented at c. 530, which is probably the most voluminous Neoproterozoic/Lower Palaeozoic syntectonic magmatism thus far recorded in E-Antarctica. The Neoproterozoic/Lower Palaeozoic structural evolution evolved in an overall sinistral transpressional setting, and thus can be correlated with the broad tectonic setting of the Mozambique Belt in Africa.

Thomas, R. J., Henjes-Kunst, F. & Jacobs, J., 1998. Pre-lamprophyre mafic dykes of the Cape Meredith Complex, West Falkland, Falkland Islands

New field, petrographic, geochemical and Rb-Sr/Sm-Nd isotope data are presented from early mafic dykes which intrude theMesoproterozoic Cape Meredith Complex, West Falkland. The dykes, which have previously regarded as being Ordovician in age, are seen in the field to cut a suite of lamprophyre sheets. New K-Ar data from biotite separates from two lamprophyres suggest a miminum age of emplacement of ~520 Ma. Our data suggests that the mafic dykes were probably intruded during the late Neoproterozoic at ~600 Ma. This new data has considerable implications for the previously published estimates of the longevity of intracontinental extension events in this part of Gondwana.

Jacobs, J., Fanning, C. M., Henjes-Kunst, F., Olesch, M. & Paech, H.-J., 1998. Continuation of the Mozambique Belt into East Antarctica: Grenville-age metamorphism and polyphase Pan-African high-grade events in central Dronning Maud Land

The about 500 km long coastal stretch of central Dronning Maud Land (DML), East Antarctica, is critical for understanding both Gondwana and Rodinia assembly. In common Gondwana reconstructions central DML lies at the potential southern extension of the Mozambique Belt. We report the first extensive geochronological study of magmatic and metamorphic rocks from the area. These new U-Pb SHRIMP zircon and Sm-Nd-data of rocks sampled during the German international GeoMaud 1995/96 expedition indicate that the oldest rocks in central DML are Mesoproterozoic in age. The crystallization ages of metavolcanic rocks were determined at c. 1130 Ma. Syn-tectonic granite sheets and plutons give ages of c. 1080 Ma, which are contemporaneous with metamorphic zircon growth at granulite facies conditions. An anorthosite intrusion and a charnockite are dated at c. 600 Ma. Subsequent metamorphism is recorded at least at two different episodes at c. 570-550 Ma and between 530 to 515 Ma. The latter metamorphic event reached granulite facies and is associated with the syn-tectonic intrusion of a granodiorite body at Conradgebirge. Initial ?_{Nd, t} values of the U-Pb dated rocks with crystallization ages around 1.1 Ga range from c. +7 and -4. These values suggest that their magmatic precursors represent variable mixtures of a primitive mantle-derived and a continental crust component that were generated within a mature island arc. Initial Nd isotope data of Cambrian meta-igneous rocks are indistinguishable from the Grenville-age rocks, probably representing partial melts of the Grenville-age basement. The occurrence of Pan-African syn-tectonic granitoids is unique in DML. The structure and shape of this body indicates that the main structural ENE-WSW trend of the region is Pan-African in age and not older as previously assumed. Some major late ductile sinistral shear zones occuring in the study area fit well in the overall sinistral transpressional setting of the Mozambique Belt. Thus, central DML very probably represents the southern continuation of the Mozambique Belt into East Antarctica. Sample locality map

Thomas, R. J., Jacobs, J. & Weber, K., 1997. Geology of the Mesoproterozoic Cape Meredith Complex, West Falkland

The geology of the Cape Meredith Complex (CMC), West Falkland is a small but important facet to the understanding of the evolution of the Grenville-aged (~1100 Ma) belts of Southern Africa and East Antarctica, and the delimitation of Pan-African (~500 Ma) reworking. The CMC is also a key to test the hypothesis that the Falkland Islands represent a rotated microplate that lay (in Gondwana) in the "Natal embayment", adjacent to the southeast African coast. The complex was mapped at a scale of 1:10 000, with selected sections at 1:2 500. The oldest rocks consist of a sequence of layered amphibolite grade mafic, intermediate and felsic gneisses, interpreted as meta-volcanic rocks, which were intruded by four phases of granitoid and later mafic and lamprophyric dykes. The rocks, particularly the granitoids, closely resemble those of the southern part of the Natal Metamorphic Province, exposed in northern Transkei (E. Cape Province, South Africa). Rotation of the Falkland microplate during Gondwana break-up through 180 brings the metamorphic fabrics of the CMC and the Transkei rocks into parallelism. However, other evidence, such as the difference in intensity of Gondwanide (Cape) deformation and the disparity in frequency of Mesozoic dolerites between East and West Falkland, may indicate, by analogy with South Africa, that the Falkland Islands themselves were not spatially adjacent in Gondwana, but were juxtaposed to their current close proximity by dextral movement along the Falkland Island Sound Fault during break-up.

Jacobs, J., Falter, M., Weber, K. & Jeßberger, E. K., 1997. ⁴⁰Ar-³⁹Ar evidence for the structural evolution of the Heimefront Shear Zone (Western Dronning Maud Land), East Antarctica

Until recently, East Antarctica was seen as a single entity that was neither significantly overprinted nor changed its shape during post Mesoproterozoic times. Now, however, there is increasing evidence of strong Pan-African (~500 Ma) reworking, in particular of the Mesoproterozoic (~1100 Ma) belts surrounding the East Antarctic Craton. The type of Pan-African overprinting ranges from collision orogenesis to simple basement exhumation. For example, along a E-W section in Dronning Maud Land, we find strong evidence of eastward-increasing Pan-African overprinting culminating in granulite metamorphism in the Lützow-Holm area. Thus, in forthcoming Gondwana and Rodinia reconstructions, East Antarctica needs to be shown as a multi-plate aggregate with variable contours through time. Heimefrontfjella in western Dronning Maud Land in parts shows a strong thermal Pan-African overprint, whereas Pan-African tectonism was apparently restricted to minor low- and high-angle shear zones. The Heimefrontfjella are cut by the Grenville-aged Heimefront Shear Zone (HSZ), separating the Vardeklettane granulite terrane from the amphibolite facies Sivorg terrane. The HSZ was intruded by many phases of mafic dykes. These dykes were subsequently deformed in steeply inclined dextral mylonite zones, interpreted as reactivated Grenvillian shear zones. In this article we present new high resolution ⁴⁰Ar-³⁹Ar data on hornblende from amphibolite mylonites within these reactivated Grenville-age mylonite zones. Three ⁴⁰Ar-³⁹Ar-ages range from 484 to 531 Ma. Their age spectra lack older age components at high heating increments. This indicates that the HSZ was reactivated during the Pan-African event causing total Ar-loss during high-temperature shearing.

Jacobs, J., Falter, M., Thomas, R.J., Kunz, J. & Jeßberger, E., 1997. ⁴⁰Ar/³⁹Ar age constraints on the structural evolution of the Mesoproterozoic Natal Metamorphic Province, SE Africa

Ten 40Ar/39Ar age spectra are presented for hornblende grains separated from mylonitised and weakly deformed amphibolites from the Mesoproterozoic (~1.2-1.0 Ga) Natal Metamorphic Province (NMP). Since the proposal of the SWEAT hypothesis, the NMP has become a crucial area in which to study Grenville-aged (locally termed "Namaquan") accretion in this part of Rodinia/Gondwana. Unlike many segments of the World’s ~1.1 belts, the NMP is particularly suited to such studies since it is devoid of a high-temperature Pan-African (~500 Ma) overprint. In this study we have dated hornblendes from within the earliest (D1) NE-directed thrust-dominated structural domains of the belt. The D1 domains are characterised by NE-directed recumbent fold and thrust nappes and a pervasive, generally S- to SW-dipping, metamorphic foliation with down-dip stretching lineations. The new data constrain the minimum age of this early event (which includes obduction of the northernmost Tugela Terrane onto the Kaapvaal Craton) to 1135± 9 Ma. In the D1 domains of the Mzumbe Terrane to the south, cooling to below ~550°C had only been attained by ~1005 Ma. We also dated amphibolites from the later (D2) structural domains which are characterised by sub-vertical sinistral mylonite belts with sub-horizontal to oblique stretching lineations. Our data show that D2 oblique shearing commenced at 1050-1035 Ma in the Mzumbe Terrane and only terminated at ~980 Ma. The youngest movements also included reactivation of major D1 structures such as the Melville Thrust (Mzumbe-Margate Terrane boundary) at ~990 Ma. Finally, indications of minor Pan-African resetting was detected during the first ~5% of the Ar-Ar spectra from mylonites of the Lilani-Matigulu Shear Zone (Tugela-Mzumbe Terrane boundary), indicating the polyphase nature of this major structure. It has been suggested that the D1 event records the early NE-SW directed arc-continent collision history of the belt whilst the D2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening. The new data place important time constraints on the major tectonic events in the NMP which appear comparable in many ways to other parts of the global "Grenvillian" orogen.

Jacobs, J. & Thomas, R.J., 1996. Pan-African rejuvenation of the 1.1 Ga Natal Metamorphic Province (South Africa). K-Ar muscovite and sphene fission track evidence

K-Ar muscovite and titanite fission track analyses were obtained from selected lithologies from the Grenville-aged (~1.1 Ga) Natal Metamorphic Province (NMP), SE Africa in order to constrain the low temperature cooling history of this critical part of Gondwana. The data revealed two periods of basement rejuvenation and exhumation at ~900 Ma and ~530 Ma respectively. Over large areas of the NMP, late-stage skeletal muscovite, dated at ~900 Ma, has overgrown the regional Mesoproterozoic fabrics. This indicates that, after magmatism and tectonics ceased at ~1000 Ma, the NMP underwent an early Neoproterozoic thermal/hydration event. Titanite fission track dates from the southern NMP scatter around 530 Ma, showing that the basement finally cooled fairly rapidly through the 300°C isotherm during Pan-African times. Since the Natal Metamorphic Province is unconformably overlain by clastic sedimentary rocks dated at ~490 Ma, the basement must have been uplifted and exhumed during the Pan-African event by ~6-10 km. Two possible uplift processes might best account for this Pan-African exhumation viz; Pan-African crustal thickening and subsequent uplift during the collision of east- and west-Protogondwana or magmatic underplating from hot mantle plumes. Although a combined collision-plume model cannot be discounted for western Dronning Maud Land, the Natal data favour the collision model, so it is proposed that the wider "Mozambique Belt" of East Africa and East Antarctica generally resulted from classical Wison Cycle processes.

Jacobs, J., Bauer, W., Spaeth, G., Thomas, R.J. & Weber, K., 1996. Lithology and structure of the Grenville-aged (~ 1.1 Ga) basement of Heimefrontfjella (East Antarctica)

The Heimefrontfjella mountains, Western Dronning Maud Land, are composed of juvenile Mesoproterozoic (Grenville-aged; ~1.1 Ga) rocks, which were reworked during the Pan-African orogeny at ~500 Ma. Three discontinuity-bounded Grenville-aged terranes have been recognized namely (from north to south) the Kottas, Sivorg and Vardeklettane terranes. The terranes contain their own characteristic lithological assemblages, though each is made up of an early supracrustal sequence of metavolcanic and/or metasedimentary gneisses, intruded by various (predominantly granitoid) suites. No older basement upon which the protoliths of these older gneisses were deposited has been recognized. In each terrane, the older layered gneisses were intruded by various plutonic suites, ranging in age from ~1150 to ~1000 Ma. The Vardeklettane terrane is characterised by abundant charnockites and two-pyroxene granulite facies parageneses in metabasites, whilst the Sivorg and Kottas terranes were metamorphosed to amphibolite facies grade. P-t estimates show that peak metamorphic conditions changed from ~ 600°C at 8 kbar in the south, to ~ 700°C at 4 kbar in the northern Sivorg terrane. Regional greenschist retrogression of high grade assemblages may be of Pan-African age. The Heimefrontfjella terranes were juxtaposed and pervasively deformed during a complex and protracted period of E-W collision orogenesis in a transpressive regime at ~1.1 Ga. This is manifest as early, gently-dipping thrust-related shear fabrics (D₁), succeeded by the initiation of an important (D₂) steep dextral shear zone (Heimefront Shear Zone - HSZ), during which the early fabrics and structures were steepened and rotated in an anticlockwise sense. The HSZ is a curvilinear structure which changes from a dextral oblique strike-slip lateral ramp in the north to a steep dip-slip frontal ramp in the south, where it forms the boundary between the Sivorg and Vardeklettane terranes. The Pan-African event is manifested as discrete, low- to medium-temperature ductile to brittle shears (D₃) and numerous K-Ar cooling ages.

Jacobs, J. & Thomas, R.J., 1994. Oblique collision at 1.1 Ga along the southern margin of the Kaapvaal continent, SE Africa

The ~ 1.1 Ga Natal Metamorphic Province (NMP) lies at the heart of a worldwide system of Grenville-age mobile belts which welded early continental fragments into the Mesoproterozoic supercontinent of Rodinia. Structural analysis of the three tectonostratigraphic terranes in Natal reveals a kinematic history characterised by prolonged NE-SW plate convergence, manifested as early thrust tectonics and later pervasive sinistral transcurrent shearing. Consequently, superimposed upon the Natal tectonostratigraphic terranes, is a kinematic subdivision into tectonic domains which are characterised by shallow, SW-dipping foliations, SW-plunging stretching lineations and NE-verging recumbent folds and by younger domains with sub-vertical shear fabrics, sub-horizontal to oblique lineations and folding about near-vertical axes. Microtextural and petrographic analyses suggest that the later shearing took place under high temperature conditions of at least 500û C. The recorded kinematic indicators suggest that early sub-horizontal compressional tectonics gave rise to tectonic thickening of the crust, progressively followed by oblique transcurrent shearing within a transpressional regime. The shearing event was in the southern arc-related terranes associated with the widespread emplacement of late-kinematic rapakivi granite-charnockite plutons, with A-type granite geochemical characteristics. This orogenic event took place around 1100 Ma during prolonged NE-SW collisional convergence along the southern margin of the stable Archean foreland, which lay to the north.

Jacobs, J., Kaul, N. & Weber, K., 1996. Thermal rejuvenation, denudation and resedimentation along the continental margin of western Dronning Maud Land during break-up of Gondwana

An ~500 km wide rim of the continental margin of western Dronning Maud Land was tectono-thermally affected during the Gondwana fragmentation. This becomes evident from apatite fission-track dating of the basement in Heimefrontfjella and Mannefallknausane and by geomorphological aspects. The continental crust of western Dronning Maud Land was thermally rejuvenated by thermal blanketing of Jurassic basalts at ~180, Ma until the Lower Cretaceous (~140 Ma) before the lava pile was eroded again. This took place in association with intense block faulting, and is in strong contrast to the sedimentation history onshore. The sedimentation on the marine plateau of western Dronning Maud Land is charaterized by 2000 m of sediments deposited at rates of >60 m/Ma during the Upper Jurassic. It is followed by an ~1000 m thick unit characterized by slower sedimentation rate from Cretaceous to Cenozoic times. Hence, intitial rifting was confined to a narrow continental rim, whilst the advanced rift stage also affected the continental margin as far inland as the Heimefrontfjella. The denuded section was probably channeled through the Jutul-Penck graben to the north and a graben system now occupied by the Endurance glacier to the west.